Borax Hydrogel Ionotronics: A Highly

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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23632−23638

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Polyvinyl Alcohol/Silk Fibroin/Borax Hydrogel Ionotronics: A Highly Stretchable, Self-Healable, and Biocompatible Sensing Platform Ningning Yang,†,∥ Ping Qi,‡,∥ Jing Ren,‡ Haipeng Yu,† Shouxin Liu,† Jian Li,† Wenshuai Chen,*,† David L. Kaplan,*,§ and Shengjie Ling*,‡,§ †

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Key Laboratory of Bio-Based Material Science & Technology, Ministry of Education, Northeast Forestry University, Harbin 150040, China ‡ School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China § Department of Biomedical Engineering, Tufts University, Medford, Massachusetts 02155, United States S Supporting Information *

ABSTRACT: The emergence of hydrogel ionotronics has significantly extended the applications of soft electronics by allowing intimate interfaces between electronic units and biological/engineered surfaces for better sensing and communication with surrounding stimuli. However, hydrogel ionotronic devices that combine high stretchability, selfhealing, good water retention, and biocompatibility are still desired. Here, we report a biocompatible ionic hydrogel made of polyvinyl alcohol, silk fibroin, and borax. In this ionic hydrogel, polyvinyl alcohol and borax offer the high stretchability and conductivity, respectively, while silk fibroin improves the stability of the hydrogel and increases water uptake by the gels. The hydrogel features strain larger than 5000%, good water retention, self-healing, and tunable conductivity and adhesive capabilities. We also demonstrate the use of the hydrogel as a sensing platform to monitor human body motion for applications in health management, soft robotics, and human−machine interfaces. KEYWORDS: polyvinyl alcohol, silk fibroin, hydrogel, ionotronics, sensor

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INTRODUCTION Hydrogel ionotronics, a smart material that combines advantages of both hydrogels and ionic conductors, has received a great deal of attention.1 In these ionic hydrogels, polymer networks stabilized by weak interactions (e.g., hydrogen bonds) make the material stretchable, while mobile ions and electrons endow electrical conductivity. The combination of mechanical performance and electronic property makes hydrogel ionotronics ideal for physical sensing applications in artificial skins, muscles, axons and cells, and triboelectric generators and for touchpads.2−4 Different polymer/salt combinations, such as polyacrylamide (PAAm)/sodium chloride (NaCl),5−7 PAAm/lithium chloride (LiCl),8−11 poly(N,N-dimethylacrylamide)/NaCl,12 polyvinyl alcohol (PVA)/borax,13 and polyacrylic acid (PAA)/alginate/ NaCl,14 have been used to construct hydrogel ionotronics. Most of these ionic hydrogels are able to stretch approximately 10 times their initial length.1 The resistivity of hydrogel ionotronics is also tunable from ∼18.2 × 104 Ω m (the value of purified water) to ∼10−6 Ω m (the value of nickel−chromium alloy), depending on the type and concentration of ions used.1 Advances in mechanical and electrical properties of these ionic hydrogels have substantially promoted applications of © 2019 American Chemical Society

these smart materials as soft electronics. However, for practical applications, a material system with high stretchability, selfhealing, good water retention, and biocompatibility is still desired. These four factors not only determine the sensitivity and long-term working stability of soft electronics but also provide conformal fits with the body, which is important for wearable applications. For example, PAAm hydrogels are highly stretchable, transparent, and easy to synthesize, but they are chemically cross-linked and thus cannot heal after failure. PVA and PAA allow the fabrication of self-healing hydrogels but are susceptible to dehydration in air. Accordingly, most of these ionic hydrogels must be sealed with elastomers, such as polydimethylsiloxane (PDMS), silicone, 3M VHB, and Ecoflex. These encapsulating materials, however, often hamper potential biomedical applications, such as implantable devices and biosensors, due to their unsatisfactory biocompatibility.15,16 The motive of this study, therefore, was to develop a highly stretchable, self-healable, water-retentive, and biocompatible Received: April 21, 2019 Accepted: May 21, 2019 Published: May 22, 2019 23632

DOI: 10.1021/acsami.9b06920 ACS Appl. Mater. Interfaces 2019, 11, 23632−23638

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic and mechanical properties of PSB hydrogels. (a) Schematic PSB hydrogel before tensile deformation. (b) Photograph of 1.0 wt % PSB hydrogel before tension deformation. (c) Schematic PSB hydrogel during tensile deformation. (d) Photograph of 1.0 wt % PSB hydrogel during tension deformation. (e) Stress−strain curves of PVA/borax (control) and PSB hydrogels.

Table 1. Mechanical Properties of Hydrogel Ionotronic Devicesa ionic hydrogels network

salt

PSB (current work) PAAM PAAM PAAM PAAM PVA PAAM PAAM PAAC/alginate PAAM PAAM PAAM PVA PVA PAAM

borax NaCl LiCl LiCl LiCl LiCl LiCl NaCl NaCl LiCl LiCl LiCl Na2B4O7 NA LiCl

elastomer 3M VHB 3M VHB 3M VHB 3M VHB 3M VHB NA 3M VHB 3M VHB 3M VHB 3M VHB 3M VHB/PDMS 3M VHB PDMS PDMS

metal

the maximum strain (%)

elastic modulus (kPa)

refs

copper copper NA silicone aluminum silicone NA NA aluminum platinum aluminum/copper platinum nickel NA

>5000 167 20 134 84 78 168 500 1000 1500 >1000 1160 700 92 >100

100 0.105 1.8 18−25 6 220 8.77 1 6.7 NA NA 4.68 NA NA NA

3 8 9 11 39 40 6 14 41 42 43 13 44 45

a

Metal refers to the contact electrode used in hydrogel ionotronic devices.

retention, self-healing ability, tunable conductivity, and adhesive capabilities.

hydrogel ionotronics to match requirements in healthcare and biomedical fields. PVA and borax were selected as hydrogel and ion conductor, respectively, because both are biocompatible and easy to dissolve in aqueous solution.17 PVA-based hydrogels have been widely studied because of their good mechanical performance. According to previous reports, these hydrogels can reach 350% strain before failure;18 however, the water retention of PVA/hydrogels is challenging to achieve in simple and efficient ways.18,19 Meanwhile, PVA/borax hydrogels, also known as slime, are highly stretchable (strain at failure higher than 700%).13 However, water retention and how to design water insoluble versions of PVA/borax hydrogels remain a challenge. PVA has been incorporated with silk fibroin (SF, a protein derived from silkworm silk fibers) in biomedical studies to produce biocompatible and biodegradable tissue-engineered scaffolds.20,21 The addition of SF in PVA hydrogels improved the stability of the hydrogels and also significantly increased water uptake by the gels.22 What’s more, SF could contribute water retention of PVA hydrogel because of its unique microstructure containing β-sheet. In this work, we constructed a type of unencapsulated hydrogel ionotronics that consisted of PVA, SF, and borax, which features ultrahigh stretchability (strains higher than 5000%), good water

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RESULTS AND DISCUSSION For an optimized protocol, Bombyx mori (B. mori) silk fibers were first degummed, dissolved, and dialyzed into SF aqueous solution (∼3 wt %) using LiBr/H2O.23,24 Then, the desired amounts of PVA and borax were directly dissolved into the SF solution to obtain a homogeneous ternary aqueous solution. The hydrogels were formed by continuous stirring at room temperature for several minutes. Fourier-transform infrared (FTIR) spectroscopy measurements confirmed that PVA molecules induced the conformational transition of SF from random coil and/or helix to β-sheet (Figure S1), serving as a cross-linker to stabilize the PVA/borax non-Newtonian fluid as a hydrogel.25 When placed in water under 60 °C for 2 h, the PVA/borax (control) hydrogel dissolved, but the 3 wt % PVA/ SF/borax (PSB) hydrogel was almost unchanged (Figure S2). As a result, the addition of SF changed the hydrogel from water-soluble to water-stable state, while preserving the nonNewtonian viscous (dilatant) behavior of the PVA/borax system.13 UV−vis spectra of PSB hydrogel and PVA/borax hydrogel showed that the transmittance of PSB hydrogels at 780 nm was 38%, lower than the PVA hydrogels (69% at 780 23633

DOI: 10.1021/acsami.9b06920 ACS Appl. Mater. Interfaces 2019, 11, 23632−23638

Research Article

ACS Applied Materials & Interfaces

Figure 2. Processability and conductivity of PSB hydrogels. (a) Photographs of PSB hydrogel and graphene/PSB hydrogel molded into arbitrary shapes. (b) Snapshots of PSB hydrogels under resiliency tests. The image was reconstructed from video snapshots (60 frames per second) and then arranged sequentially from left to right. (c) Photograph of luminescent 1.0 wt % PSB hydrogels under extreme stretching. The PSB hydrogels were stretched into fibers in air using a porous tensile frame. (d) Photograph to illustrate that luminescent 1.0 wt % PSB fibers were solidified and dried in air by external infrared irradiation. (e) Fluorescence microscopy image of 1.0 wt % PSB fibers. (f) Cross-sectional scanning electron microscopy (SEM) image of the 1.0 wt % PSB fibers. (g) Photographs to show changes in resistance of the 1.0 wt % PSB hydrogel under mechanical stretching from 0% (the insert image) to 1000%. (h) Experimental setup for recording the resistance changes of the 1.0 wt % PSB hydrogel under mechanical testing. (i) Electric resistance−strain curve of PSB/graphene hydrogel (1:0.003).

Figure 3. Self-healing and adhesion ability of PSB hydrogel. (a) Two fractured hydrogels were brought into contact, and the crack healed completely. (b,c) Photographs of self-healed hydrogel (b) before and (c) after stretching. (d) Resistance changes of 1.0 wt % PSB hydrogel after cutting and self-healing. (e) Process where a 1.0 wt % PSB hydrogel was cut and self-healed. (f−i) Photographs (f,g) and corresponding force− displace curves (h,i) to illustrate adhesion of 1.0 wt % PSB hydrogel with an artificial hand (f) before and (g) after infrared light illumination.

thickness of the hydrogels. For simplicity, the hydrogels reported here were labeled according to the solid content of SF. For example, 1.0 wt % PSB hydrogel means that the solid

nm) (Figure S3). PSB hydrogels appeared uniform in terms of structure, and the transparency could be modulated by changing the mass ratio between PVA and SF, as well as the 23634

DOI: 10.1021/acsami.9b06920 ACS Appl. Mater. Interfaces 2019, 11, 23632−23638

Research Article

ACS Applied Materials & Interfaces

Figure 4. PSB hydrogel sensing platform. (a) Photographs to demonstrate that a PSB/graphene (1:0.003 w/w) hydrogel was used to monitor leg bending movement. (b) Resistance response of 1.0 wt % PSB hydrogel during bending motion of the human knee. (c) Resistance response of 1.0 wt % PSB hydrogel during five fingers making different gestures. (d) Resistance response of 1.0 wt % PSB hydrogel when holding a pen that is falling. (e) Experimental setup for recording the resistance of the conductive 1.0 wt % PSB hydrogel attached to the body armor surface. (f) Resistance change of 1.0 wt % PSB hydrogel impacted by a hammer.

(Figure 2a) and fluorescence dyes (Figure 3a), were easily added into the hydrogels before gelation to add function. The graphene addition endowed PSB hydrogels with good thermal absorbing ability (Figure S6) without influencing conductivity or mechanical properties (Figure S7), while the conductivity changed corresponding to changes in temperature. The dilatant characteristics of PSB hydrogels offer a facile route for generating fibrous ionotronics. Luminescent PSB hydrogels were stretched into homogeneous PSB fibers in the air by using a porous tensile frame (Figure 2c and Movie 1). These luminescent PSB fibers could be directly solidified and dried in air by external infrared irradiation (Figure 2d). The excellent water-uptake capability of the PSB system allows the PSB fibers to be turned back into fibrous hydrogels through absorbance of water vapor (Figure S8). The as-fabricated PSB fibers were present in small diameters of 20−80 μm and meterscale lengths (Figure 2e,f). An in situ experiment was designed to record the changes in resistance when the hydrogels were stretched (Figure 2g,h), and the resistance increased slowly when the stretching was under 300%, rising rapidly after 400% (Figure 2i and Movie 2). The conductivity of PSB hydrogels could be retained even in extreme stretching conditions, because the stretchability of PSB hydrogels is entropic and causes changes in structure of the polymer network and water molecules, negligibly impacting ionic conductivity. For example, the resistance of 1.0 wt % PSB hydrogels (65 mm × 20 mm × 2 mm) increased from 0.277

content of SF is 1.0 wt %. In all experiments, the mass ratio between PVA and borax was fixed to 75:2. Tensile mechanical measurements were utilized to estimate the mechanical properties of the PSB hydrogels (Figure 1). For the freshly prepared hydrogels, all the splines with different SF contents feature ultrahigh stretchability. For example, the 1.0 wt % hydrogel stretched more than 3000% their initial length without failure, displaying much higher strain to failure than most reported ionic hydrogels, such as PAAm, PDMS, and PVA hydrogels (Table 1). Although the stretchability of PSB hydrogels was weakened by increased SF content, the strain at failure still maintained 350% even when the SF content was up to 3.0 wt %. Such surprising stretchability originates from weak bonds between the PVA chains (Figure 1a−d), which leads to the formation of a viscoelastic non-Newtonian rheological (referred to as dilatant) behavior.13 A typical feature of dilatant hydrogels is the intrinsic elastic and viscous nature characterized by shear thickening behavior, where shear viscosity increases with applied shear stress.26 As a result, PSB hydrogels were flexible at low-speed movement and thus can be molded into arbitrary shapes yet provide rigidity under high-speed movement to resist piercing by stabbing blows by sharp objects (Figure 2a,b). In addition, PSB hydrogels had good elastic recovery under cyclic stretching (Figure S4) and good compliant properties without separation between the hydrogel and the finger surface (Figure S5). Furthermore, other functional components, such as graphene 23635

DOI: 10.1021/acsami.9b06920 ACS Appl. Mater. Interfaces 2019, 11, 23632−23638

Research Article

ACS Applied Materials & Interfaces

required. In this process, the skin significantly wrinkled from the tearing force applied on the hydrogel layer. In addition, the interface adhesive capability was tunable through facile and safe strategies. For example, the hydrogels could be easily removed after 10 min of infrared light illumination. In contrast, the adhesive capability could be enhanced by increased environmental humidity, with a tearing force of 0.015 MPa. Figure 4a illustrates the potential applications of PSB hydrogels for physical sensing platforms.37,38 A graphenedoped PSB hydrogel-based sensor with a width of 1 cm was fixed on a kneecap and connected with a wireless multimeter. The resistance changed during marching and was recorded in real-time (Figure 4b). The resistance of the PSB hydrogel sensors decreased dramatically when stretching occurred and varied synchronously with the bending and release of the leg. Although the resistance change rose slowly in this process due to the dehydration phenomenon, the response rate of hydrogels maintained 0.5 s, a distinct signal change above 10% with excellent repeatability. The PSB hydrogels also adhered to fingers to sense motion. In this prototype, the hydrogel was directly connected to a computer coupled with a multichannel dc resistance tester. The corresponding resistance changes in each finger could be recorded in real-time. This setup enabled the possibility to monitor finger deformation and to distinguish distinct finger gestures, simultaneously. For example, the PSB hydrogels successfully sensed the whole process of hand grabbing. A more complex hand motion, a continuous change of hand gestures of “5-0-6-8-3-0-5-4-2-6”, was further designed to test the sensitivity of the PSB hydrogel system. As demonstrated in Figure 4c, the resistances of each finger sensor changed synchronously with the finger movements. Meanwhile, no signal interference from adjacent fingers was detected in the process. The PSB hydrogel successfully monitored the complete process of hand grabbing. The resistance of the five fingers changed when grabbing an object (Figure 4d and Movie 3). Profiting from non-Newtonian behavior, these PSBs are resistant to high-speed and strong impacts. As a proof-ofconcept demonstration (Figure 4e), PSB hydrogels were fixed on a bulletproof jacket and connected with a multichannel dc resistance tester. Strong and continued hammer impacts were applied on the hydrogel ionotronics. The resistance of the hydrogels remained constant and no electrical failure was detected in response to these impacts (Figure 4f), showing potential applications in advanced armor sensing systems.

kΩ to 0.192 MΩ after 10-fold extension. By contrast, most physical sensing platforms that consist of polyethylene terephthalate,27 polycarbonate,28 and polyethylene naphthalate29 electrically fail at a tensile strain of 1−5%. The water content (here defined as a ratio of the sample weight after being placed over time to the initial weight, 25 °C, 65% relative humidity) in PSB hydrogel has an essential impact on the mechanical and electrical properties of hydrogel ionotronics. Here, PSB hydrogels showed improved water retention compared to PVA/borax hydrogels (control hydrogel without SF) (Figure S9a). After placement in an open environment for 2 days (25 °C, 65% relative humidity), the PSB hydrogels maintained 75% water content and retained excellent fracture strain above 4300%. After 4 days, the PSB hydrogels retained a water content of 60%. However, the fracture strain dropped dramatically to 200% (Figure S9b). Meanwhile, changes in resistance versus strain of the PSB hydrogels with 75% water content had a broader strain response compared to the PSB hydrogels with 60% water content (Figure S9c). These water retention results corresponded to the FTIR spectroscopy measurements, demonstrating that a transition of SF from random coil and/or helix to β-sheet (Figure S1) serves as a cross-linker to stabilize the PVA/borax non-Newtonian fluid as a hydrogel. Moreover, the conductivity of PSB hydrogels decreased from 0.63 S/cm to 0.42 S/cm when the water content of the PSB hydrogels dropped from 75 to 60% (Figure S9d). Unless mentioned, all the experiments reported in the present work were taken within 2 days after material preparation, and thus the water content was always above 80%. Self-healing ability is desired for multifunctional hydrogels, which can be realized by many strategies, such as metal−ligand coordination, hydrophobic association, π−π stacking, hydrogen bond interactions, or combinations of these mechanisms.30−32 Here, two key reformable interactions make the PSB hydrogels self-healable. The first one is the hydrogen bonding generated by the PVA molecules, which provides numerous −OH groups along the main chains. In the wet state, these hydrogen bonds are reformable after damage.33 The second interaction originates from the dynamic diolborate ester bonds formed between PVA and borax.34 These two self-healable interactions mean that the hydrogels can be reshaped and reused even when they are partially damaged. When two hydrogels were mixed (e.g., rhodamine B and alcian blue introduced to produce the colors shown) and pulled to a strain of 1000%, the hydrogels remained stable and did not fracture (Figure 3a−c). The self-healing of the hydrogel also occurred during object deformation. As shown in Figure 3d,e, a cut PSB hydrogel can be reconnected, and the conductivity was restored to the original level. Such self-healing ability was maintained after this cut−reconnect processes carried 10 times, without affecting the stability of the electrical conduction. We further assessed the adhesive capability of the PSB hydrogels with artificial skin. A strong interface between the physical sensing platform and biological or engineered surfaces is essential for practical applications.35,36 When the PSB hydrogel was attached onto the surface of an artificial hand, it deformed together with the finger, even when the hand was fully clenched. The good bonding between hydrogels and hand skin was further confirmed by example (Figure 3f,g) and quantitative (Figure 3h,i) tearing experiments. To tear a 1.0 wt % PSB hydrogel from the artificial skin, 0.015 MPa stress was

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CONCLUSIONS In summary, a highly stretchable, water-retentive, self-healable, and biocompatible hydrogel ionotronics was developed using PVA, SF, and borax as raw materials. In this ternary system, PVA and borax offered high stretchability and conductivity, while SF stabilized the hydrogel and increased water uptake by the gels. The resultant hydrogel features ultrahigh stretchability (strain larger than 5000%), water retention, self-healing, and tunable conductivity and adhesion. Finally, we demonstrated how this hydrogel could be used as a sensing platform to monitor surrounding body motion for applications in healthcare monitoring, soft robotics, and human−machine interfaces.

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METHODS

Preparation of SF Solution. The silkworm cocoons (10 g) were degummed by boiling in the deionized water with 0.5% (w/w) 23636

DOI: 10.1021/acsami.9b06920 ACS Appl. Mater. Interfaces 2019, 11, 23632−23638

ACS Applied Materials & Interfaces

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NaHCO3 for 30 min to obtain degummed fibers. The degummed fibers were rinsed thoroughly with the deionized water and dried at room temperature for 3 days. Degummed fibers were then dissolved in the 9.3 M LiBr solution at a ratio of 1:10 (s/l) at 60 °C for 1 h and then followed by dialyzing with distilled water for 2−3 days until the pH of the water became constant over a period of few hours. The SF solution about 4% was obtained from liquid supernatant after centrifugation at 6000 rpm. Fabrication of PSB Hydrogel Ionotronics. Three hydrogels corresponding to the SF contents of 0.4, 0.8, and 1.0 wt % were prepared by mixing PVA (7.5 g dissolved in 15 mL distilled water), SF (5, 10, and 15 ml 4% SF solution, respectively), and borax (0.2 g dissolved in 5 ml distilled water) and stirred into a homogeneous solution. PSB hydrogel ionotronics were fabricated with or without 0.3 wt % graphene addition on the base of hydrogel (SF content of 1.0 wt %), and similarly, the colorful hydrogels were obtained with the addition of rhodamine (0.5 mg/g). Water Retention Testing of PSB Hydrogels. The hydrogel was shaped into a spherical sample and placed in an open environment (25 °C, 65% relative humidity). The weight of the spherical sample was measured per 24 h, and then water retention was calculated by the ratio of the sample weight after being placed over time to initial weight. Mechanical Testing of PSB Hydrogels. For the tensile tests, 20 mm segments of the PSB hydrogels were fixed on an Instron 5966 machine (Instron, Norwood, USA). The strain rate was fixed at 500 mm/min. To record the resistance changes of the PSB hydrogels during tensile deformation, the ends of the conductive PSB hydrogels were attached to copper foils to connect with a digital multimeter (CEM, DT-9989), and the resistance variations were recorded in realtime at a time resolution of 1 s. Sensing Measurements of the PSB Hydrogel Platform. The ability of the PSB hydrogel sensors was tested by a multichannel touchscreen digital meter (Tektronix DMM6500). To monitor human activities, hydrogel-based sensors were attached onto the knee and fingers and connected to the Tektronix DMM6500 to realize synchronous detection. The resistance values were recorded by a computer at the time resolution of 0.5 s. Characterization of Materials. The surface and cross sections of the PSB hydrogel fibers were observed by fluorescence microscopy (Leica DMi8, Germany) and SEM (JEOL JSM-6010PLUS/LA) at an acceleration voltage of 5 kV. To prevent electrical charging, samples were coated with a 5 nm thick gold layer before observation. FTIR measurements were carried out by an FTIR Spectrometer (Spectrum Two, PerkinElmer, UK). For each measurement, 64 interferograms were co-added and Fourier-transformed employing a Genzel-Happ apodization function to yield spectra at a nominal resolution of 4 cm−1.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.C.). *E-mail: [email protected] (D.L.K.). *E-mail: [email protected] (S.L.). ORCID

Shouxin Liu: 0000-0002-0491-8885 Wenshuai Chen: 0000-0003-4428-1505 David L. Kaplan: 0000-0002-9245-7774 Author Contributions ∥

N.Y. and P.Q. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge National Natural Science Foundation (no. U1832109), Shanghai Pujiang Program (18PJ1408600), and the starting grant of ShanghaiTech University for support of this work. We also thank the NIH (P41EB002520, U01EB014976), the AFOSR and CℏEM, SPST of ShanghaiTech University (#EM02161943) for support for various aspects of the work.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06920. PSB hydrogels stretched into fibers (2× fast forward) (AVI) Resistance response of stretching hydrogel (4× fast forward) (AVI) Resistance change of PSB hydrogel when holding an object (3× fast forward) (AVI) Fourier infrared spectra; photographs; UV-vis spectra; elastic recovery property; compliant property; thermal absorbing ability; mechanical property and conductivity; water-uptake capability; and water retention rate of the hydrogel samples (PDF) 23637

DOI: 10.1021/acsami.9b06920 ACS Appl. Mater. Interfaces 2019, 11, 23632−23638

Research Article

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DOI: 10.1021/acsami.9b06920 ACS Appl. Mater. Interfaces 2019, 11, 23632−23638